Mechanism of Action: Plan B vs Ella
Plan B (levonorgestrel) primarily works by preventing or delaying ovulation when taken before the LH surge, while Ella (ulipristal acetate) has a broader window of action and can delay ovulation even when follicles are larger and closer to rupture 1, 2.
Plan B (Levonorgestrel) Mechanism
According to the FDA-approved label, Plan B works mainly by stopping the release of an egg from the ovary 1. The mechanism is:
- Primary action: Prevents or delays ovulation when follicles are ≤14 mm
- Timing-dependent efficacy: Most effective when taken before the LH surge begins
- No post-fertilization effects: Does not prevent implantation or disrupt an established pregnancy 2, 3
The key limitation is that levonorgestrel becomes ineffective once follicles exceed approximately 14 mm in diameter, which typically occurs 1-2 days before ovulation 4.
Ella (Ulipristal Acetate) Mechanism
Ulipristal acetate is a selective progesterone receptor modulator that acts as a progesterone antagonist 5. Its mechanism includes:
- Primary action: Delays or inhibits ovulation even with larger follicles (up to 18 mm)
- Wider window of effectiveness: Maintains efficacy 3-5 days post-intercourse, when levonorgestrel loses effectiveness 6, 4
- No post-implantation effects: Does not disrupt established pregnancy or prevent implantation 2, 3
Key Clinical Differences
Timing of effectiveness:
- Both work similarly within 72 hours of intercourse 6
- UPA maintains superior efficacy between 72-120 hours when levonorgestrel effectiveness declines 6, 7
Follicular size at action:
- Levonorgestrel: Effective only on follicles <14 mm
- Ulipristal acetate: Effective on follicles up to 18 mm 4
This biological difference explains why UPA is more effective overall—it can prevent ovulation closer to the actual time of follicular rupture, providing protection during a larger portion of the fertile window 2, 4.
Important Clinical Caveat
Neither medication works by preventing implantation. Despite older FDA labeling suggesting implantation prevention as a possible mechanism, current research clearly demonstrates that both levonorgestrel and ulipristal acetate at emergency contraception doses have no significant endometrial effects and cannot prevent implantation of a fertilized egg 2, 3. This misconception has been used to restrict access and causes significant public confusion 8.